Plankton are any drifting organisms, animals, plants, archaea, or bacteria, that inhabit the pelagic zone, water not close to the bottom or shore, of oceans, seas, or bodies of fresh water. Plankton are defined primarily by their ecological niche rather than phylogenetic or taxonomic classification. They provide a crucial source of food to larger, more familiar aquatic organisms such as fish and cetacea.
Types of plankton include: phytoplankton, which are autotrophic, prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis; zooplankton, which are small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton; and bacterioplankton, bacteria and archaea, which play an important role in remineralising organic material down the water column, a conceptual column of water from surface to bottom sediments used chiefly for environmental studies evaluating the stratification or mixing (e.g. by wind induced currents) of the thermal or chemically stratified layers in a lake, stream or ocean. Some of the common parameters analyzed in the water column are: pH, turbidity, temperature, salinity, total dissolved solids, various pesticides, pathogens and a wide variety of chemicals and biota.
Phytoplankton, also known as microalgae, are similar to terrestrial plants in that they contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean, where sunlight penetrates the water. Phytoplankton are the foundation of the aquatic food web, the primary producers, feeding everything from microscopic, animal-like zooplankton to multi-ton whales. When too many nutrients are available, phytoplankton may grow out of control and form harmful algal blooms. These blooms can produce extremely toxic compounds that have harmful effects on marine, fowl, and mammals, including humans.
Phytoplankton growth depends on the availability of carbon dioxide, sunlight, and nutrients. Phytoplankton, like land plants, require nutrients such as nitrate, phosphate, silicate, and calcium at various levels depending on the species. Some phytoplankton can fix nitrogen and can grow in areas where nitrate concentrations are low. They also require trace amounts of iron which limits phytoplankton growth in large areas of the ocean because iron concentrations are very low. Other factors influence phytoplankton growth rates, including water temperature and salinity, water depth, wind, and what kinds of predators are grazing on them.
Knowledge of phytoplankton size and taxonomic composition is critical to characterizing biogeochemical cycles and quantifying carbon export. It is essential for predicting the ocean's response to future climate change. Shifts in species size or taxonomic composition, for example, may affect zooplankton grazing, and the packaging of material into fecal pellets, which will in turn impact carbon export from, or recycling within, the water column. As phytoplankton community structure, i.e., the various species and types of phytoplankton in the area, can be highly variable in space and time, its characterization requires sensors that can monitor continuously, and be deployed at multiple fixed locations or used on tethered or autonomous underwater vehicles (AUVs).
The need for in situ sensors has received increasing recognition by the oceanographic community in the past few years. Programs such as ORION/OOI and NOPP have enhanced scientific ocean observing capabilities, both in near-shore and open-ocean environments. While most ocean sensing platforms are equipped with a fluorometric sensor for chlorophyll a (chl a), this pigment (or a derivative) is found in all microalgae and thus cannot be used to discriminate between different phytoplankton taxa or to discern cell size.
Significant progress has been made on the development of in situ, i.e., on site, flow cytometric instruments that are capable of automated characterization of phytoplankton communities, however these instruments requiring cabling to a shore-based observatory. Satellite-based ocean color sensors provide critical information on phytoplankton biomass across broad swaths of the sea but characterization of community composition by satellite is difficult and generally limited to species with unique optical signatures.
A trend appears to be emerging toward in situ instruments capable of obtaining greater detail in both phytoplankton morphology and spectroscopy. Bulk optical spectroscopy measurements suffer from overlap of the spectra of many taxa. Mathematical methods exist to separate limited numbers of different fluorescent species from one another. However, the most general and accepted approach to disentangling the optical spectra of differing phytoplankton is to isolate them for individual measurement, either by cytometry or by imaging.
Flow cytometry on naturally-occurring phytoplankton provides both light scattering (size-related information) and laser-excited fluorescence emission intensity (pigment-related information). Light scattering is linked by theory to the morphology and optical constants of phytoplankton, although the mathematics cannot be inverted to determine exact morphology from light scattering. The fluorescence measured by a flow cytometer from natural phytoplankton is likewise limited because not enough of the excitation spectrum of the pigments is usually sampled. Many of the currently-available spectral fluorescence-based instruments, such as the Mini-Tracka II (Chelsea Instruments, UK), the Algae Online Monitor (Photon Systems Instruments, Czech Republic) and the Algae Online Analyzer (bbe Moldaenke, Germany; Beutler et al. 2002) suffer from poor discrimination abilities due to the limited number of excitation wavelengths.
Imaging provides more information about size and shape than light scattering can provide. Many reports on flow cytometry also provide details of microscopic analysis as a standard for comparison. In one embodiment, flow cytometry may be augmented with rapid imaging if the intention is to obtain information on dominant species. Conversely, fast automated imaging alone has not been shown to discriminate among a wide range of phytoplankton and other particles. Imaging may be coupled with fluorescence for the purpose of classifying or identifying plankton. Full-spectrum absorption spectroscopy has been shown to be a useful tool for classification. However, this technique does not give phytoplankton size and gives relatively limited information on phytoplankton community composition.
Sensors that can be deployed broadly on mobile or fixed platforms that give detailed information on phytoplankton size and species composition have remained elusive. What is needed is a new instrument for in situ discrimination of phytoplankton size and community composition that is compact, inexpensive, and has low power requirements.